DESCRIPTION

The ucred family of functions is used to manage user credential
structures (structucred) within the kernel.
The crget() function allocates memory for a new structure, sets its
reference count to 1, and initializes its lock.
The crhold() function increases the reference count on the credential.
The crfree() function decreases the reference count on the credential.
If the count drops to 0, the storage for the structure is freed.
The crshared() function returns true if the credential is shared. A
credential is considered to be shared if its reference count is greater
than one.
The crcopy() function copies the contents of the source (template)
credential into the destination template. The uidinfo structure within
the destination is referenced by calling uihold(9).
The crcopysafe() function copies the current credential associated with
the process p into the newly allocated credential cr. The process lock
on p must be held and will be dropped and reacquired as needed to
allocate group storage space in cr.
The crdup() function allocates memory for a new structure and copies the
contents of cr into it. The actual copying is performed by crcopy().
The crsetgroups() function sets the cr_groups and cr_ngroups variables
and allocates space as needed. It also truncates the group list to the
current maximum number of groups. No other mechanism should be used to
modify the cr_groups array except for updating the primary group via
assignment to cr_groups[0].
The cru2x() function converts a ucred structure to an xucred structure.
That is, it copies data from cr to xcr; it ignores fields in the former
that are not present in the latter (e.g., cr_uidinfo), and appropriately
sets fields in the latter that are not present in the former (e.g.,
cr_version).
The cred_update_thread() function sets the credentials of td to that of
its process, freeing its old credential if required.

RETURNVALUES

crget(), crhold(), crdup(), and crcopysafe() all return a pointer to a
ucred structure.
crshared() returns 0 if the credential has a reference count greater than
1; otherwise, 1 is returned.

USAGENOTES

As of FreeBSD 5.0, the ucred structure contains extensible fields. This
means that the correct protocol must always be followed to create a fresh
and writable credential structure: new credentials must always be derived
from existing credentials using crget(), crcopy(), and crcopysafe().
In the common case, credentials required for access control decisions are
used in a read-only manner. In these circumstances, the thread
credential td_ucred should be used, as it requires no locking to access
safely, and remains stable for the duration of the call even in the face
of a multi-threaded application changing the process credentials from
another thread.
During a process credential update, the process lock must be held across
check and update, to prevent race conditions. The process credential,
td->td_proc->p_ucred, must be used both for check and update. If a
process credential is updated during a system call and checks against the
thread credential are to be made later during the same system call, the
thread credential must also be refreshed from the process credential so
as to prevent use of a stale value. To avoid this scenario, it is
recommended that system calls updating the process credential be designed
to avoid other authorization functions.
If temporarily elevated privileges are required for a thread, the thread
credential can by replaced for the duration of an activity, or for the
remainder of the system call. However, as a thread credential is often
shared, appropriate care should be taken to make sure modifications are
made to a writable credential through the use of crget() and crcopy().
Caution should be exercised when checking authorization for a thread or
process perform an operation on another thread or process. As a result
of temporary elevation, the target thread credential should never be used
as the target credential in an access control decision: the process
credential associated with the thread, td->td_proc->p_ucred, should be
used instead. For example, p_candebug(9) accepts a target process, not a
target thread, for access control purposes.